This invention relates to optical waveguide multimode to single mode transformers and to applications of such transformers in transferring optical signals from a laser to an optical fiber.
Advances in optical amplifiers based on either erbium doped fiber or Raman amplification require increased power launched into a single mode optical fiber. Single mode lasers used as pumps for optical amplifiers can provide launched powers of approximately 200 mW at a wavelength of 980 nm. The requirement of single mode operation limits the width of the laser active layer to 3-4 microns. This results in high optical power density at the mirror of the laser and consequently mirror damage. Higher powers are provided by arrays of single mode pump lasers, with each laser operating at a slightly different wavelength. The different wavelengths can be combined into a single mode fiber using wavelength division multiplexing techniques. This approach is limited in the number of lasers that can be practically operated at the same time, as well as by the finite spectral width of the pumping band of optical amplifiers.
Significantly larger output powers can be obtained from broad area lasers, which typically have transverse dimensions of 10 to 100 microns, and without any structures that might stabilize the mode. It thus cannot operate in the lowest transverse mode. The output is divergent, 30-40 degrees in the vertical direction, because the mode in the laser is confined to a very thin active region and it is diffracted at the laser facet. Devices with an active region width of 100 microns typically can provide power output of up to two watts. However, a limitation of a broad area laser is the filamentary nature of the laser field, which in the absence of any mode stabilizing structure breaks up into filaments resulting in non-uniform optical power density across the mirror. This type of laser may be used as a pump in amplifiers or lasers that do not require coupling to single mode fibers. In fact coupling to a single mode fiber is not possible because of the multimode nature of broad area lasers.
Use of a pointed tapered waveguide for coupling a laser to an optical fiber has been proposed in xe2x80x9cEfficient coupling of a semiconductor laser to an optical fiber by means of a tapered waveguide on silicon,xe2x80x9d Y. Shani, C. H. Henry, R. C. Kistler, K. J. Orlowsky, and A. Ackerman, Appl. Phys. Lett., vol. 55, pp. 2389-2391. xe2x80x9cIntegrated Optic Adiabatic Devices on Silicon,xe2x80x9d Y. Shani, C. H. Henry, R. C. Kistler, R. F. Kazarinov, and K. J. Orlowsky, IEEE Journal of Quantum Electronics, vol. 27, No. 3, pp. 556-566, March 1991, discloses a waveguide, which may support a few modes and is adiabatically narrowed to a single-mode waveguide and then widened again, adiabatically, to its former size. It is stated that the possible higher order states are stripped off in the narrow waveguide region so that only the fundamental mode remains.
It is known that suppression of filamentary operation results in significantly higher power output but mismatch between the mode of the laser and that of the optical fiber remains. Aspects of the present invention address two problems, stabilization of a broad area laser output to provide a single mode optical signal and efficient coupling of single mode optical signals to a single mode optical fiber.
According to the invention, a method of optical radiation mode transformation comprises introducing multimode optical radiation into a waveguide structure providing adiabatic transformation of a fundamental mode of said multimode optical radiation to single, fundamental mode optical radiation. Preferably, multimode optical signals are output from a laser into a waveguide structure having a lateral waveguiding dimension that varies along the length of the waveguide structure in a manner effecting transformation of a fundamental mode of said multimode optical signals to a single, fundamental mode, and outputting said single, fundamental mode optical signals to an optical fiber supporting transmission of said single, fundamental mode signal.
An optical waveguide mode transformer embodying the invention comprises an optical waveguide structure including a high refractive index core layer between lower refractive index cladding layers, said core layer including a wide input waveguide section coupled by an intermediate waveguide section to an output waveguide section, said intermediate waveguide section including a tapered region; said wide input waveguide section having a width to accept a multimode, including a fundamental mode, light input, said output waveguide section having a width to support a single mode light output comprising said fundamental mode, and said tapered region having a taper length enabling adiabatic transfer of said fundamental mode of said multimode light from said wide input waveguide section to said output waveguide section. Advantageously in the input and intermediate waveguide sections, the core layer is contoured to provide an effective index step portion to control lateral waveguiding of light propagated along the waveguide structure, and the output waveguide section includes an output waveguide portion having a real index step between the core layer and the cladding layers, and is functional to output a light beam having similar vertical and horizontal divergences. This enhances coupling of the light beam to a single mode optical fiber.
A preferred embodiment of the invention employs two types of solid state optical waveguide, to provide, on the one hand, efficient coupling to the laser by using a ridge waveguide to implement an effective index step structure, and on the other hand, to provide efficient coupling to a single mode optical fiber by using a waveguide with a real index step. A real index step may be obtained by using as the core of the waveguide, a material having an index of refraction that is larger than the index of refraction of the cladding layer. This can be done by surrounding the material of the core layer with a material having a lower index of refraction. This is in contrast to structures in which the effective index of refraction is increased, in the lateral direction, by a confinement structure fabricated in or on the cladding layer. Such a waveguide can be formed by etching the core layer to define an upstanding rib along the length of the core, without introducing any differences in the materials of the core layer of the waveguide. The transition of the fundamental mode of the optical beam from one type of waveguide to the other is accomplished adiabatically in order to minimize optical losses. In addition, a cross-sectional plane is defined at which the modes of the two types of waveguide are closely matched. Such a circuit is referred to as a mode transformer, which functions to transform a multimode optical signal to a single (fundamental) mode optical signal. Advantageously, the real index waveguide section can be structured to output a light beam having similar vertical and horizontal divergences, enhancing coupling of the light beam to a single mode optical fiber. In an alternative implementation, the mode transformer utilizes a solid state waveguide, which has a tapered P-doped silicon dioxide core on a silicon nitride layer to obtain an effective increase in the index of refraction.
A mode transformer embodying the invention may be employed in conjunction with a solid state laser to implement an external cavity solid state laser. A solid state laser is coupled to output a divergent beam, multimode optical signal, including a fundamental mode, to an optical waveguide mode transformer. The solid state laser and the optical waveguide mode transformer have anti-reflective coated neighboring end surfaces; the laser has a high reflectivity coated opposite surface and the mode transformer has a low-reflectivity coated opposite end surface. The optical waveguide mode transformer comprises low refractive index cladding material on either side of a high refractive index core layer and the core layer may include a lengthwise extending ridge having a width that functionally changes along its length to accept the multimode optical signal output from the solid state laser to transform the fundamental mode of the multimode optical signal to a single fundamental mode optical signal, without significant loss of intensity of the fundamental mode signal. This fundamental mode signal is propagated to an output section of said optical waveguide mode transformer, terminating at the low reflectivity coated end of the mode transformer, which supports single mode propagation of said fundamental mode optical signal.
It should be emphasized that a mode transformer embodying the invention does not function to convert the order of the transverse mode of optical radiation; specifically, it does not convert higher order modes to the fundamental mode. The mode transformer functions, in one direction of propagation, to change the field distribution of the fundamental mode of a multimode input signal from a large spatial extent to a small one, and vice versa in the opposite direction of propagation, with minimal loss of fundamental mode beam intensity, i.e. adiabatically. As the dimension of the transverse mode is reduced by reducing the width of the taper section of the solid state waveguide, higher order modes cannot be supported any more and are radiated out (stripped) from the sides of the waveguide core.
In application to amplifiers pumping single mode fibers, a lens formed at the input end of the fiber results in higher launched power. In other applications, where high power launched into solid state waveguide is of interest, a cylindrical lens (e.g. a piece of optical fiber) is placed between the laser and the cleaved input face of the waveguide of the mode transformer. The latter application relies on broad area lasers. The effect of the lens is to collimate laser output in a vertical direction. A mode transformer embodying the invention can be used, in combination with the laser and a cylindrical lens, to stabilize the laser output in the fundamental mode and to couple it efficiently into a single mode fiber.